The present invention relates to a gas flow measuring apparatus for measuring and detecting the amount of intake air flowing through the air intake pipe in an internal combustion engine, for example, wherein the output signal of this measuring apparatus is used by the internal combustion engine control unit as one of the signals for detecting the state of operation, said internal combustion engine control unit serving to electronically control the amount fuel injected, the ignition timing, etc., in the internal combustion engine.
The electronic control unit for vehicle engines, for example, measures and detects the amount of intake air as one of the elements of engine performance to determine the state of engine operation. The air flow measuring device is used as a means to measure the amount of intake air. This air flow measuring device is located in the air intake pipe for the engine and heat-wire gas flow measuring device are often used as mass flow meters to detect the electronic measuring signal.
FIG. 1 shows a typical example of the circuit for this air flow measuring apparatus. A bridge circuit 6 includes a heater 2 whose heat is removed by the flow of air, a heat sensitive element 3 for measuring the temperature of the air, and balancing resistances 4 and 5. A power source current controlling transistor 9 is controlled by the output of an OP amplifier 8 in such a way that the potential difference between a connecting point A of the heater 2 and resistance 4 and a connecting point B of the heat sensitive element 3 and resistance 5 is a value which is determined by a constant-voltage source 7. The value of the current flowing to the heater 2 at this time is detected as a potential difference between both ends of the resistance 4, and this detected voltage is picked up as measuring output signal.
In the case where this air flow measuring apparatus is used to electronically control the internal combustion engine, however, dust adheres to the heater 2 because it is left for a long time in the flow of air to be measured. When dust adheres to the heater 2, the heat releasing effect of the heater 2 which is caused by the flow of air changes and the relationship between the amount of air flow and output voltage changes accordingly, thereby making it difficult to obtain an output signal which represents the correct amount of air flow.
When dust adheres to the heater, the heat transmitting characteristic of the heater to the air generally becomes worse. Even in the case where the amount of air which acts on the heater 2 is equal, the heater 2 covered with dust is heated to the same temperature by less current, thereby balancing the bridge circuit 6.
As the dust becomes thicker on the heater 2, the value of the output voltage decreases even when the amount of air flow is maintained. When the amount of air flow is to be increased using this measuring output signal, therefore, a signal which represents an amount of air flow smaller than the actual one will be obtained. When the air flow measuring apparatus whose heater 2 is covered with dust is used as the apparatus for measuring the amount of intake air in the internal combustion engine, therefore, backfiring is caused in winter and the heater 2 in the apparatus thus becomes further covered with dust. The amount of air to be measured becomes still smaller than the actual one and the amount of fuel injected is controlled less than required, thereby causing backfiring again.
Heating the heater 2 to high temperature and burning off the dust, that is, has been considered as a means for solving this problem. Even if the heater is heated to a high temperature, the dust cannot be burned off completely but will sinter and stick still more to the heater. Therefore, this burning off is not an acceptable solution for obtaining a correct measuring output.
Air flow measuring apparatus which are more flexible, particularly, against dust are disclosed by U.S. Pat. Nos. 4,393,702, 4,399,697 and 4,384,484. When these apparatus, however, the temperature of the heater is transmitted to the heat sensitive element and the temperature change of the heater changes the temperature of the heat sensitive element, which comprises the bridge circuit, thereby slowing their response characteristic. When any of these devices is employed in the electronic control unit for vehicle engines, it becomes necessary to add various corrections and carry out an extremely complicated correct operation, and the like, in the course of changing engine operation.